1. Field of the Invention
This invention relates to a thermal head drive circuit in a recording device such as a facsimile recorder or a printer which uses a thermal head.
2. Description of the Prior Art
A recording device which uses either a heat-sensitive recording sheet or a transfer type heat-sensitive recording medium to thermally record data is extensively employed for facsimile recording or the like. In general, a recording device of this type uses a thermal head as its recording head, which has a linear array of heat generating units or elements. The thermal head provides thermal energy for printing. Therefore, the thermal head suffers from a difficulty arising from the required heat thus provided with the result that the generated print is degraded in quality.
Especially, the print is adversely affected by heat accumulated during high speed recording. The heat which is electrically produced in the heat generating elements is utilized for printing and is thereafter radiated away through the base plate of the thermal head. However, when the thermal head is driven at high speed, for instance when a printing cycle is 10 millisecond or less, then the printing operation starts before the heat from the preceding operation is sufficiently dissipated through radiation, and accordingly heat is accumulated in the heat generating elements. As a result, the heat generating elements are not uniform in temperature during recording, and the printed dots are different in size and not uniform in density.
A recording device which records data with a transfer type-heat sensitive recording medium 11 piled on a recording sheet (ordinary sheet) 12 as shown in FIG. 1 suffers from the difficulty that heat generated by a thermal head 13 spreads out in the medium 11, thus thermally affecting the recording of the next line or the recording of the adjacent picture elements on the same line.
A recording device which performs a printing operation by causing one elongated heat generating structure to generate heat in only one part of the structure suffers from the drawback that heat is accumulated in the recording of the first half of one line, thus adversely affecting the recording of the second half. This will be described with reference to FIG. 2. Two groups of lead wires 15 and 16 are alternately connected at their first ends to an elongated heat generating structure 14 forming the thermal head of the recording device in such a manner that the lead wires are arranged at equal intervals. The lead wires 15 have the other ends connected to the respective parallel signal output terminals of a shift register driver 17. The lead wires 16 have their other ends alternately connected through diodes 18 to first and second common electrodes C1 and C2. Printing data 19, which are half of the data required for recording one line, are supplied to the shift register driver 17. The data are obtained by extracting bits at intervals of two bits from the complete data. After the data have been set in the shift register driver 17, a voltage is applied to the first common electrode C1. As a result, portions of the heat generating structure 14 which are between the lead wires 15 and the lead wires 16 connected to the first common electrode C1 are driven. These portions will hereafter be referred to as "heat generating elements". Recording is performed by the heat generating elements which are electrically energized to generate heat. After the first half of one line has been recorded, data required for recording the second half are set in the shift register driver 17. These data are the remaining half of the complete data. After the data have been set in the shift register driver 17, a voltage is applied to the second common electrode C2 to drive the remaining heat generating elements of the heat generating structure 14. Similarly in the second step, recording is performed by the heat generating elements which are electrically energized to generate heat. The second half of one line is recorded a very short time after the first half has been recorded. The heat which is generated in the recording of the first half of the line adversely affects the recording of the second half, as a result of which the quality of picture is lowered.
With the heat generating structure 14 as described above, a further problem arises in that a leakage current may pass through some of the heat generating elements depending on the signal which is applied to the shift register driver 17. For instance in the case where, under the condition that the voltage is applied to the first common electrode C1 and the leftmost lead wire 15 in FIG. 2 is not grounded but the next lead wire 15 is grounded, then a leakage current 21 flows to the lead wire 15 thus grounded as indicated by the dotted line in FIG. 2. Accordingly, each of three heat generating elements through which the leakage current flows generates heat the amount of which is one-ninth (1/9) of the amount of heat which is generated during printing. This leakage current heat also adversely affects the quality of print.
In order to overcome the drawback that the quality of print is lowered by the adverse effect of thermal energy, attempts have been made to adjust the voltage applied to the thermal head or the width of a pulse applied thereto, to thereby set the amount of thermal energy to a suitable value for each line. One example of such a thermal head drive circuit is as shown in FIG. 3. In the circuit, a counter 25 counts the black (printing) bits for every line which are included in the printing data 24 which is supplied to a thermal head 23. The counter 25 supplies a control signal 26 to a thermal energy control circuit 27 in correspondence to the count value. The thermal energy control circuit 27 is for instance a pulse voltage setting circuit or a pulse width setting circuit, which, when the thermal head 23 records a line next to the line which the counter 25 has counted, adjusts pulses 28 applied to the heat generating elements.
However, such uniform control for the entire thermal head cannot eliminate the above-described difficulties satisfactorily. Sometimes the control locally increases or decreases the temperature of the heat generating elements, thus lowering the quality of print. In the case of a recording device in which heat diffuses in the direction of the line (main scanning direction), similarly to the spreading shown in FIG. 1, even if heat accumulation is corrected for every line, its effect is not sufficient to overcome the diffusion.